sigma11-049. 355KB Jun 04 2011 12:10:52 AM

Symmetry, Integrability and Geometry: Methods and Applications

SIGMA 7 (2011), 049, 13 pages

Symmetries in Connection Preserving Deformations⋆
Christopher M. ORMEROD
La Trobe University, Department of Mathematics and Statistics, Bundoora VIC 3086, Australia
E-mail: C.Ormerod@latrobe.edu.au
Received January 31, 2011, in final form May 18, 2011; Published online May 24, 2011
doi:10.3842/SIGMA.2011.049
Abstract. We wish to show that the root lattice of B¨acklund transformations of the qanalogue of the third and fourth Painlev´e equations, which is of type (A2 + A1 )(1) , may be
expressed as a quotient of the lattice of connection preserving deformations. Furthermore,
we will show various directions in the lattice of connection preserving deformations present
equivalent evolution equations under suitable transformations. These transformations correspond to the Dynkin diagram automorphisms.
Key words: q-Painlev´e; Lax pairs; q-Schlesinger transformations; connection; isomonodromy
2010 Mathematics Subject Classification: 34M55; 39A13

1

Introduction and outline

Discrete Painlev´e equations are non-autonomous second order difference equations admitting
the Painlev´e equations as continuum limits [25]. These equations arise as contiguity relations
for the Painlev´e equations [10], from discrete systems arising in quantum gravity [9], reductions
of discretizations of classically integrable soliton equations [8] and various recurrence relations
for orthogonal polynomials [6]. There are many ways in which they may be considered integrable
such as the singularity confinement property [7], solvability via associated linear problems [24]
and algebraic entropy [2].
A fundamental result of Okamoto is that the group of B¨acklund transformations of the
Painlev´e equations are of affine Weyl type [19, 20, 18, 21]. The work of the Kobe group has
remarkably shown the discrete Painlev´e equations admit similar representations of affine Weyl
groups [17]. This understanding was enhanced by the pioneering result of Sakai [26], who
extended the work of Okamoto on the associated surface of initial conditions to the discrete
Painlev´e equations [26]. A valuable insight of this work is that the B¨acklund transformations
and the discrete Painlev´e equations should be considered to be elements of the same group.
It has been well established that the Painlev´e equations admit Lax representations. The
underlying theory behind these Lax representations is the theory of isomonodromy [12, 10, 11].
The first evidence that the q-difference Painlev´e equations admitted Lax representations can
be found in the work of Papageorgiou et al. [24] where it was shown that a q-discrete version of
the third Painlev´e equation arises as the compatibility condition of two systems of q-difference
equations, written as two n × n matrix equations of the form

Y (qx) = A(x)Y (x),
Y˜ (x) = R(x)Y (x),

(1a)
(1b)

where A(x) is some n × n rational matrix in x and q ∈ C is a fixed constant such that |q| =
6 1
and the evolution denoted by tildes coincides with the evolution of the discrete Painlev´e equa⋆
This paper is a contribution to the Proceedings of the Conference “Symmetries and Integrability of
Difference Equations (SIDE-9)” (June 14–18, 2010, Varna, Bulgaria). The full collection is available at
http://www.emis.de/journals/SIGMA/SIDE-9.html

2

C.M. Ormerod

tion. If (1a) and (1b) form a Lax representation of a Painlev´e equation, then the compatibility
condition determines the evolution of that discrete Painlev´e equation [24].
The q-difference analogue of the concept of an isomonodromic deformation was proposed

later on by Jimbo and Sakai [13]. The system admits two symbolic solutions,
Y0 (x) = Yˆ0 (x)D0 (x),
Y∞ (x) = Yˆ∞ (x)D∞ (x),
where Yˆ0 and Yˆ∞ are series expansions around 0 and ∞ respectively and D0 and D∞ are
composed of q-exponential functions [27]. Since Y0 and Y∞ are both fundamental solutions, they
should be expressible in terms of each other, which gives rise to the connection matrix, C(x),
specified by
Y0 (x) = Y∞ (x)C(x).

(2)

For regular systems of linear q-difference equations, such as the linear problem in [13], the
solutions converge under more or less general conditions specified in [3]. However, in the irregular
case Y0 (x) and Y∞ (x) do not necessarily define holomorphic functions, hence, we take this
definition is taken to be symbolic here. For irregular systems of q-difference equations we have
that there exists at least one convergent solution, however, in general, to describe the solutions
one would be required to incorporate a q-analogue of the Stokes phenomena [28].
In this study, we take a connection preserving deformation to be characterized by a transformation of the form (1b). In fact, the original Lax representation proposed in [24] also defines
a connection preserving deformation in the sense of Jimbo and Sakai [13].
In previous studies, we considered a lattice of connection preserving deformations, which

is a lattice of shifts of the characteristic data which defines the connection matrix [23, 22].
However, we struggled to incorporate how the various Dynkin diagram automorphisms [17]
manifested themselves in the theory of the connection preserving deformations. We find that
many connection preserving deformations are copies of the same evolution equations. This
induces a natural automorphism on the level of the evolution equations. These automorphisms
correspond to Dynkin diagram automorphisms.
We demonstrate the role of the Dynkin diagram automorphisms in a study of the associated
linear problem for q-PIII and q-PVI , which possesses a group of B¨acklund transformations of
(1)
type W ((A2 + A1 )(1) ), which is the case of P (A5 ) in Sakai’s notation. This is an interesting
case as the symmetries have been nicely studied before [15] and that the two equations possess
the same surface of initial conditions [26] and the same associated linear problem where the
two systems are different directions on the lattice of connection preserving deformations [23].
This lattice of connection preserving deformations admits a natural group of automorphisms of
order six, corresponding naturally to the sub-group of B¨acklund transformations generated by
the Dynkin diagram automorphisms. This advances our understanding of the relation between
the associated linear problems for q-difference equations and their symmetries.

2

Birkhof f theory for irregular dif ference equations

Before specifying the form of the solutions, let us fix some notation required to specify the
solutions. We specify some of the building blocks used in the Galois theory approach to the
study of systems of linear q-difference equations [27, 29]. Let us consider the q-Pochhammer

Symmetries in Connection Preserving Deformations

3

symbol, given by

k−1
Q



(1 − aq n ) if 0 < k < ∞,



 n=0
(a; q)k =
1
if k = 0,


∞

Q


(1 − aq n ) if k = ∞,

n=0

where we will also use the notation
n
Y
(a1 , . . . , an ; q)k =
(ai ; q)k .

i=1

We specify the Jacobi theta function as
X n
q ( 2 ) xn ,
θq (x) =
n∈Z

which satisfies
xθq (qx) = θq (x).
It is also useful to define the q-character as
eq,c (x) =

θq (x)θq (c)
,
θq (xc)

which consequently satisfies the pair of equations
eq,c (qx) = ceq,c (x),

eq,qc (x) = xeq,c (x).

Using the above building blocks, it is an elementary task to construct solutions to any first order
linear q-difference equation, hence, we may transform any system of q-difference equations of the
form (1a), where A(x) is rational, to a system of q-difference equations where A(x) is polynomial.
Hence, without loss of generality, we may let
A(x) = A0 + A1 x + · · · + Am xm .
We isolate the set of points in which A(x) is singular, i.e., where det A(x) = 0, by fixing the
determinant as
det A(x) = κxL (x − a1 ) · · · (x − ar ).
We will call a problem of the form (1a) regular if A0 and Am are diagonalizable and invertible and
irregular if it is not regular. The formal expansion for regular problems may be computed under
certain non-resonance conditions specified by the work of Birkhoff [3]. However, to pass to the
irregular theory, we must refer to the work of Adams [1], which was subsequently rediscovered
by Birkhoff and Guenther [4].
Theorem 2.1. The linear problem (1a) admits two fundamental series solutions, Y0 (x) and
Y∞ (x), specified by the expansions



eq,λi (x)
Y0 (x) = Y0 + Y1 x + Y2 x2 + · · · diag
,
(3a)
θ(x)li




eq,κi (x)
Y−1 Y−2
′
Y∞ (x) = Y0 +
,
(3b)
+ 2 + · · · diag
x
x
θ(x)ki
where κi , λi , li and ki must satisfy the condition that if li = lj (ki = kj ) then λi 6= λj q p

(κi 6= κj q p ) for any integer p 6= 0.

4

C.M. Ormerod

In general, we are required to choose Y0 and Y0′ to diagonalize A0 and Am respectively. More
details on the process of finding these solutions may be found in the work of Adams [1]. In this
irregular setting the solutions found do not necessarily define holomorphic functions [5]. Once
these solutions are defined, we may specify the connection matrix via (2). However, we are
required to take (2) to be a symbolic definition at this point.
To characterize the set of deformations considered, we introduce the characteristic data [23,
22]. The characteristic data consists of the variables defining the asymptotic behavior of the
solutions at x = 0 and x = ∞, defined by (3a) and (3b), and the roots and poles of A(x). We
denote this data by


κ1 , . . . , κ n a 1 , . . . , a r
M=
.

λ1 , . . . , λ n
However, while this study extends previous works [23, 22], a task remains to fully describe the
set of solutions by incorporating a q-analogue of the Stokes phenomenon [28]. That is, in order
to fully mirror the theory of monodromy, we are also required to include data that encodes the
Stokes phenomenon for systems of linear q-difference equations [28].
We now take a deformation of (1a) to be defined by (1b), where it is easy to see that Y˜ (x)
must satisfy


˜ Y˜ (x),
Y˜ (qx) = R(qx)A(x)R(x)−1 Y˜ (x) = A(x)
(4)

˜
which defines A(x).
We take this new linear problem to be associated with a new connection
˜
˜ . If the fundamental solutions
matrix, C(x),
and a new set of characteristic data, denoted M
satisfy the conditions
Y˜0 (x) = R(x)Y0 (x),
Y˜∞ (x) = R(x)Y∞ (x),

(5a)
(5b)

˜
then it is clear from (2) that (5a) and (5b) implies that C(x)
= C(x). In the regular case,
a specific case of the converse implication was presented in the work Jimbo and Sakai’s work [13],
however, for the case of irregular systems of q-difference equations, where the fundamental
solutions are not necessarily holomorphic functions, the analogous converse implication is not
so clear.
˜
If we do take R(x) to be rational, as reported in a previous work [23], we can say that M
has an altered set of characteristic constants. On the level of the series solutions of Adams [1],
Birkhoff and Guenther [4], a transformation of the form (1b) may
• change the asymptotic behavior of the fundamental solutions at x = ∞ by letting κi →
q n κi ;
• change the asymptotic behavior of the fundamental solutions at x = 0 by letting λi → q n λi ;
• change the position of a root of the determinant by letting ai → q n ai .

We cannot deform these variables arbitrarily, as there is one constraint we must satisfy; if we
consider the determinant of the left and right hand side of (1a) for the solution Y = Y0 at x = 0,
it is easily shown that
Y Y
Y
κi (−ai ) =
λi .
(6)

˜ . This is a constraint on how we may deform the characteristic
This must also hold true for M
data.

Symmetries in Connection Preserving Deformations

5

Conversely, let us suppose that λi and κi are changed by a multiplication by some power of q,
−1 are both rational functions, meaning that the expansions
˜ ∞ D∞
˜ 0 D−1 and D
then D
0
R(x) = Y˜0 (x)Y0 (x)−1 ,

R(x) = Y˜∞ (x)Y∞ (x)−1 ,

(7)

present two different expansions around x = 0 and x = ∞ respectively. Furthermore, taking
˜
a determinant of (4) as it defines A(x)
specifies that R(x) satisfies the equation
˜
det R(qx)
det A(x)
=
.
det R(x)
det A(x)
An alternative characterization of (4) is given by the way in which we have two ways to calculate Y˜ (qx):
˜
Y˜ (qx) = A(x)R(x)Y
(x),

Y˜ (qx) = R(qx)A(x)Y (x),

leading to the compatibility condition
˜
A(x)R(x)
= R(qx)A(x).
This type of condition appears throughout the integrable literature [24, 13]. Given a deformation
˜ , the above constitutes enough information to determine R(x)
of the characteristic constants, M
and also determine the transformation R(x) induces [23].
At this point, we wish to outline the idea of the symmetries of the associated linear problem
as one finds a host of deformations of the characteristic constants, which we call the lattice of
connection preserving deformations, presented in a previous work [23]. This is an idea distinct
from the work of Jimbo and Sakai [13] in that there is no one canonical deformation, but rather
a family of them. If one considers the set of all possible moves, one may endow this with a lattice
structure of dimension r + 2n − 1. In addition to the above translations one expects a certain
number of symmetries to be natural in this setting. One of the most natural groups that should
be allowed to act on the set of characteristic data should be the group of permutations on the r
roots [23].
We wish to examine an additional structure that may be obtained from the lattice. We
consider two systems to be equivalent if there exists a transformation that maps the evolution
of one system to the other and vice versa. A classical example would be P34 and PII , which are
related via a Miura transformation. We will find that the transformations induced by many of
the directions in the lattice of connection preserving deformations are equivalent, splitting the
group into classes of equivalent systems, related to each other by a transformation. Conceptually,
one may think of these transformations that permute the directions of the lattice as rotations
on the lattice, and that the fact that the lattice admits these rotations as being a property of
the lattice.

3

The associated linear problem

We will begin by specifying the properties of the linear system:
• The matrix A(x) is chosen so that
det A(x) = κ1 κ2 x(x − a1 )(x − a2 ).
• The solution at x = 0 is given by



eq,λ1 (x)
0
.
Y0 (x) = Y0 + Y1 x + Y2 x + · · ·
0
eq,λ2 (x)/θq (x)
2

(8)

6

C.M. Ormerod
• The solution at x = ∞ is given by



Y−1 Y−2
eq,κ1 (x)/θq (x)2
0
+ 2 + ···
Y∞ (x) = I +
.
0
eq,κ2 (x)/θq (x)
x
x

(9)

The determinant and the various other asymptotics specify that
A(x) = A0 + A1 x + A2 x2 .
This would ordinarily leave us to define twelve variables, however, the normalization of the
solution at x = ∞, the determinantal constraint and the solutions at 0, in addition to the
relation
κ1 κ 2 a 1 a 2 = λ 1 λ 2
leave three variables to be chosen. If one choose the variables arbitrarily, the evolution equations
are not guaranteed to be nice. So the question remains, how does one choose these variables
so that the resulting evolution equations appear in some sort of canonical manner? It is here
that the continuous case lends incredible insight: if one refers to the work of Jimbo, Miwa and
Ueno [12, 10, 11], it is apparent that the relevant variables, almost invariably are chosen so that
one variable is the zero of the upper right entry of the matrix, one variable encapsulates the gauge
freedom and one more variable is associated with the evaluation of the diagonal elements at the
root of the upper right entry. That is to say that if we define a notation for the individual entries
of A(x) as A(x) = (ai,j (x)), then a1,2 (y) = 0. Furthermore, the gauge freedom is encapsulated
by letting
a1,2 (x) ∝ w(x − y).
There remains one more variable to choose, and this is done so that


z1 0
,
A(y) =
∗ z2
where there is a determinantal constraint linking z1 to z2 . In the continuous case, it is typical
that this constraint is that A(x) is traceless at x = y, giving z1 = −z2 . In our case, we will
require that det(A(y)) = z1 z2 , which specifies one degree of freedom, chosen to be represented
by a variable, z. Remarkably, this choice seems to be canonical in that the resulting evolution
equations, after one performs the appropriate connection preserving deformations, appear to be
in a symplectic form.
This completely specifies a matrix parameterization of the form
!
κ1 ((x − y)(x − α) + z1
κ2 w(x − y)
A(x) =
,
κ1
(γx + δ)
κ2 (x − y + z2 )
w
where α, γ and δ are functions of y and z specified by the constraints. As it is quite easily
derivable from the above constraints, and variants have appeared before [16, 23, 22], we simply
list the parameters as
−z1 κ1 + (y − z2 ) κ2 + λ1
,
γ = −2y − α + a1 + a2 + z2 ,
yκ1
y (y − a1 )
(yα + z1 ) (y − z2 )
,
z1 =
,
z2 = z (y − a2 ) .
δ=
y
z
α=

Symmetries in Connection Preserving Deformations

7

Now that we have a parameterization for A(x), to find the relevant connection preserving deformations using (7), we require knowledge of the fundamental solutions. To do this, we simply
substitute (8) and (9) into (1a) and solve for Yi . It is sufficient to compute the first few terms
for computation of the connection preserving deformations specified later on. We will start with
(k)

the expansion around x = ∞. Let us first specify notation as Yk = yi,j , then if we consider
0 = Y∞ (qx) − A(x)Y∞ , at order x−3 we have


(−1)

xκ1 q(y + α) − (q − 1)y1,1


(−1)


x − wκ2 − κ1 y1,2


q


0=




(−1)
(−1)
(−1) 

xκ1 wy2,1 − qγ
wκ2 qy − qz2 − (q − 1)y2,2 − qγκ1 y1,2 
qw

qw

specifying Y−1 completely. We could go on to calculate Y−2 and Y−3 , however, in the interest of
keeping this account elegant, we shall stop. However, it suffices to say that the series solution
may be found to any order.
To specify the solution around x = 0, we first need to define Y0 . Of course, there is a nonuniqueness in how we define Y0 , as given any matrix that diagonalizes A0 , multiplication on
the right by any invertible diagonal matrix results in another matrix that diagonalizes A0 . We
choose a relatively simple matrix to simplify some of our calculations, namely we choose Y0 to
be


wy
wyκ2
Y0 =
.
−zy + y + za2 (−zy + y + za2 ) κ2 + λ1
Although it is quite simple to compute Y1 , Y2 and so on, the computations become increasingly
verbose, hence, we will only list the above terms.

4

The symmetries of the associated linear problem
and the B¨
acklund transformations

Now that everything has been defined, we may study the lattice of connection preserving deformations. We have six variables in total that we may deform and one constraint, specified
by (6). This gives us a five basis elements to find. To reduce some of the workload, let us
consider the most basic of transformation: notice that if we multiply the fundamental solutions
by R(x) = xI, where I is simply the identity matrix, we may absorb this into the factors,
˜ i = qλi . Since this does not change the
D0 and D∞ , by letting the κi → κ
˜ i = qκi and λi → λ
relative gauge freedom, this does not change the definition of y, hence, y˜ = y, z˜ = z and w
˜ = w.
˜
We find it convenient to introduce the notation




κ1 κ2 a 1
qκ1 qκ2 a1
Tκ1 ,κ2 ,λ1 ,λ2 :
: w, y, z →
: w,
˜ y˜, z˜ ,
λ 1 λ2 a 2
qλ1 qλ2 a2
where w
˜ = w, y˜ = y and z˜ = z in accordance with the transformation. We need to choose four
separate generators.
To reduce workload once more, it is clear, in accordance with the parameterization, there is
a symmetry we may exploit. As mentioned above, we expect to be able to permute the roots in
˜
a natural manner, if we appropriately define y˜, z˜ and w.
˜ We note that A(x)
= A(x) if we let




y − a2
κ 1 κ2 a 1
κ 1 κ2 a 2
Sa1 ,a2 :
: w, y, z →
: w, y, z
.
λ1 λ2 a 2
λ 1 λ2 a 1
y − a1

8

C.M. Ormerod

We choose the following

κ1 κ 2
Ta1 ,λ1 :
λ1 λ2

κ1 κ 2
Ta2 ,λ1 :
λ1 λ2

κ1 κ 2
Tκ1 ,λ1 :
λ1 λ2

κ1 κ2
Tκ2 ,λ2 :
λ1 λ2

additional elements


a1
: w, y, z →
a2


a1
: w, y, z →
a2


a1
: w, y, z →
a2


a1
: w, y, z →
a2

to represent the five dimensional lattice:

κ1 κ2 qa1
: w,
˜ y˜, z˜ ,
qλ1 λ2 a2

κ1 κ2 a 1
: w,
˜ y˜, z˜ ,
qλ1 λ2 qa2

qκ1 κ2 a1
: w,
˜ y˜, z˜ ,
qλ1 λ2 a2

qκ1 κ2 a1
: w,
˜ y˜, z˜ ,
λ1 qλ2 a2

where w,
˜ y˜ and z˜ is to be determined in each case. These four elements and Tκ1 ,κ2 ,λ1 ,λ2 form
a basis for the lattice of connection preserving deformations.
Theorem 4.1. The transformations Ta1 ,λ1 , Tκ1 ,λ1 and Tκ1 ,λ2 are induced by transformations of
the linear problem
Y˜ (x) = Ra1 ,λ1 (x)Y (x),
respectively, where


Y˜ (x) = Rκ1 ,λ1 (x)Y (x),

qy (y − a1 )
x + qy − qa1 +
z (a2 − y)
x − qa1




Ra1 ,λ1 = 


 q (y(z − 1) − za2 ) (y(z − 1) + a1 − za2 )
−
wz (x − qa1 ) (y − a2 )


(y(z − 1) − za2 ) κ2 wκ2
x+

yκ1
κ1 
,
Rκ1 ,λ1 (x) = 


y(z − 1) − za2
1
wy


(y(z − 1) − za2 ) κ2 − λ1 wκ2
x+

yκ1
κ1 
.
Rκ1 ,λ2 (x) = 
 (y(z − 1) − za ) κ − λ

2
2
1
1
wyκ2

Y˜ (x) = Rκ1 ,λ2 (x)Y (x),




qwy (y − a1 )

z (x − qa1 ) (y − a2 ) 

 ,

y−a1
x + qy yz−za2 − 1 


(10a)

x − qa1

(10b)

(10c)

Proof . In considering Ra1 ,λ1 , since κ1 and κ2 are unchanged, we know that
 
1
−1
˜
,
Y∞ (x)Y∞ (x) = I + O
x
this, coupled with the determinantal relation specify that
det Ra1 ,λ1 =

x
,
x − a1

hence, we seek a parameterization of the form
Ra1 ,λ1 (x) =

xI + R0
.
x − qa1

˜
We consider the compatibility in determining Y˜ (qx) to be given by R(qx)A(x) = A(x)R(x).
Taking the residue at x = a1 gives us R0 completely in terms of the untransformed variables,
namely, we obtain a representation of Ra1 ,λ1 given by (10a).

Symmetries in Connection Preserving Deformations

9

To compute Rκ1 ,λ1 (x), the difference in asymptotic behavior of Y∞ (x) with Y˜∞ (x) is used as
we notice


x
0
˜
˜
−1
−1
−1
˜ ∞ (x)D∞ (x) Yˆ = Yˆ∞ (x)
Yˆ −1 .
Y˜∞ (x)Y∞ (x) = Yˆ∞ (x)D
∞
0 1 ∞
˜
The leading terms of Yˆ∞ (x) and Yˆ∞ (x) are both I, hence, Rκ1 ,λ1 is given by a formal expansion
of the form

 

1
1 0
Rκ1 ,λ1 (x) = x
+ R1 + O
,
0 0
x
where we use the fact det Rκ1 ,λ1 (x) = x to bound the order of the expansion. Where the
previous calculation allowed a simple derivation of the entries of Ra1 ,λ1 (x) via the computation
of the residue, there is an added difficulty in computing Rκ1 ,λ1 . We are required to look at va˜
rious combinations of the entries of the compatibility relation, A(x)R
κ1 ,λ1 (x) = Rκ1 ,λ1 (qx)A(x).
However, it is not a very difficult calculation. The result is given by (10b). The derivation
of Rκ1 ,λ2 follows the same pattern as the previous case.

While we have mainly used the compatibility relation to define the entries of the R matrices,
it is also possible to expand the Y˜∞ (x)(Y∞ (x))−1 to higher orders to find R. However, however
one may determine the entries, it becomes abundantly clear that we have many more relations
than we need to define the entries of R in each case. The remaining relations may be used to
express y˜, z˜ and w
˜ in terms of y, z and w.
Theorem 4.2. The effect of the transformations, Ta1 ,λ1 , Tκ1 ,λ1 and Tκ1 ,λ2 , are specified by the
relations
qy (y − a1 ) κ1
z˜ (˜
z a2 κ2 + qλ1 )
,
z˜z =
,
q (˜
z − 1) κ1
(y − a2 ) κ2
w (˜
z − 1) (qyκ1 − zκ2 )
z (˜
z a2 κ2 + qλ1 )
qya1 κ1 + qzλ1
: w
˜=
,
y˜y =
,
z˜ =
,
zκ1
q (˜
z − 1) κ1
qy 2 κ1 − yzκ2
z˜ (˜
z a2 κ2 + qλ1 )
,
: w
˜ = −w (˜
z − 1) ,
y y˜ =
q (˜
z − 1) κ1
qκ1 (za1 a2 κ2 + (a1 − y) λ1 )
z˜z = −
.
κ2 (a2 (κ2 − qyκ1 ) + λ1 )

Ta1 ,λ1 : w
˜ = w (1 − z˜) ,
Tκ1 ,λ1
Tκ1 ,λ1

y˜y =

Note that inverses of all these transformations are easily computed in the form presented.
We now state that the lattice of connection preserving deformations as
L = hTκ1 ,κ2 ,λ1 ,λ2 , Ta1 ,λ1 , Ta2 ,λ1 , Tκ1 ,λ1 , Tκ1 ,λ2 i ∼
= Z5 ,
the elements above commute and form a basis for this lattice.

5

The multiple correspondences to q-PIII and q-PIV

There are two problems we wish to address in this section that are not covered by previous
studies [23]. Firstly, the dimension of the lattice of connection preserving deformations is five,
whereas the root lattice of type (A2 + A1 )(1) is three. We also wish to look at how the various
correspondences between the two lattices gives rise to the Dynkin diagram automorphisms.
One expects that the set of translational B¨acklund transformations embed naturally in this
lattice of connection preserving deformations. We introduce a similar notation on the variables

10

C.M. Ormerod

for the affine Weyl group of type (A2 + A1 )(1) , as found in the work of Kajiwara et al. [14]. We
(1)
define one reflection, s1 , and one rotation, σ0 , which generates a group of type A2 , given by
s0 :
σ0 :





b0 b1
f 0 , f1 , f 2
b2



b1 b2
f 1 , f2 , f 0
b0

→



→





1
b0

b2 b0
b0
b2

b1 b 0



f 0 , f1

b1
f 0 , f1 , f2 ,

b0 + f 0
1 + b0 f 0



, f2



1 + b0 f 0
b0 + f 0



,

where the other reflections may be defined to be s1 = σ0 ◦ s0 ◦ σ02 and s2 = σ02 ◦ s0 ◦ σ0 . We also
define some extra generators, σ1 and w0 , to be




b0 b 1 1 1 1
b0 b1
,
, ,
σ1 :
f 0 , f1 , f 2 →
b2
b2
f0 f1 f2



b0 b1 b0 b1 (b2 b0 + b2 f0 + f2 f0 )
b0 b 1
,
w0 :
f 0 , f1 , f2 →
b2
b2
f2 (b0 b1 + b0 f1 + f0 f1 )

b1 b2 (b0 b1 + b0 f1 + f0 f1 ) b1 b2 (b0 b1 + b0 f1 + f0 f1 )
,
,
f0 (b1 b2 + b1 f2 + f1 f2 ) f0 (b1 b2 + b1 f2 + f1 f2 )
where we define one more operator, w1 = σ1 ◦ w0 ◦ σ1 . It is a general result of [14] that the
generators of G = hs0 , s1 , s2 , w0 , w1 , σ0 , σ1 i satisfy all the relations of the extended affine Weyl
group of type (A2 + A1 )(1) .
Following [14], there are four translational components, T0 , . . . , T3 , where T0 ◦ T1 ◦ T2 = I,
which generates a three dimensional lattice. These are
T0 = σ0 ◦ s 2 ◦ s 1 ,

T1 = σ0 ◦ T1 ◦ σ02 ,

T2 = σ02 ◦ T1 ◦ σ0 ,

T3 = σ 1 ◦ w 0 .

√
We note that if we define1 qc2 = f0 f1 f2 and b0 b1 b2 = q, then the subgroup, hs0 , s1 , s2 , σ0 i
√
preserves c, while T3 maps c → qc.
The task remains to make a correspondence between the root lattice and the lattice of connection preserving deformations. However, given the theory established by previous authors [14],
T0 , T1 and T2 present isomorphic evolutions, hence, we should be able to determine six correspondences between the connection preserving deformations and the lattices. We seek to recover
these isomorphisms from the connection preserving deformation standpoint.
Let us list explicitly give the representation of T0 :
( √
)


qb0 √b1q ˜ ˜ ˜
b0 b 1
T0 :
f 0 , f1 , f2 →
f0 , f1 , f2 ,
b2
b2
where
f˜0 f0 =

qc2 (b


√ 
qc2 b0 f˜0 + q
f˜1 f1 =
√
,
f˜0 b0 + qf0

+ f1 )
,
f1 (b1 f1 + 1)
1

and f˜2 is defined by f˜0 f˜1 f˜2 = qc2 . With regards to T0 , we can immediately make a correspondence with Ta−1
as they are of similar forms. By letting
1 ,λ1
y=−
1

a1
,
b0 f 0

z=−

f1
,
b1

b20 =

a1
,
a2

b21 = −

a 2 κ2
,
λ1

a 1 a 2 κ1
c2 = √
,
qλ1

(11)

This definition is trivially different to that of [14], this is to make a full correspondence between the connection
preserving deformation above and the translational components of the group of B¨
acklund transformations.

Symmetries in Connection Preserving Deformations

11

we obtain a correspondence between the evolution in y and z and f0 and f1 . The correspondence
sought between Tκ1 ,λ1 and T0 may be specified by
y=−

a 1 b 0 b1 f 0 f 1
,
c2 q 3/2

z=−

f0
,
b0

b20 = −

a 2 κ2
,
λ1

b21 = −

qλ1
,
a 1 κ2

a 1 a 2 κ1
c2 = √
, (12)
qλ1

while the correspondence between Ta−1
and T0 is specified by
2 ,λ1
y=−

a 2 b1
,
f1

z=

qa1 κ1
,
b0 b 1 f 0 f 1 κ 2

b20 = −

qλ1
,
a 1 κ2

b21 =

a1
,
a2

a 1 a 2 κ1
c2 = √
.
qλ1

(13)

Lastly, there are secondary correspondences between the given lattices. Although (11) gives one
way of mapping the evolution of Ta−1
to T0 , we also have that
1 ,λ1
y = −a2 b0 f0 ,

z=−

1
,
b1 f 1

b20 =

a1
,
a2

b21 = −

a 2 κ2
,
λ1

λ1
c2 = √
q qa1 a2 κ1

(14)

gives another correspondence that inverts the value of c. We have similar correspondences that
invert c for Ta−1
and Tκ1 ,λ1 . The correspondences between the evolutions is summarized in
2 ,λ1
Table 1.
Table 1. We list the way in which the lattice of connection preserving deformations may align with the
root lattice. The first three fix c while the last three invert c.

T0

T1

T2

T3

Ta−1
1 ,λ1

Tκ1 ,λ1

Ta−1
2 ,λ1

Tκ1 ,λ2

Tκ1 ,λ1

Ta−1
1 ,λ1

Tκ1 ,λ2

Ta−1
2 ,λ1
−1
Ta1 ,λ1

Ta−1
2 ,λ1
Ta−1
1 ,λ1

Tκ1 ,λ1

Tκ1 ,λ2

Tκ1 ,λ1

Ta−1
2 ,λ1

Tκ−1
1 ,λ2

Tκ1 ,λ1

Ta−1
2 ,λ1

Ta−1
1 ,λ1

Tκ−1
1 ,λ2

Ta−1
2 ,λ1

Ta−1
1 ,λ1

Tκ1 ,λ1

Tκ−1
1 ,λ2

If we consider the element that changes correspondences between (11) and (12):
˜b2 = a1 = q = b2 ,
0
2
a2
b20 b21

˜b2 = − a2 κ2 = b2 ,
0
1
λ1

c˜2 = c2 ,

similarly
qc2
a1
f˜0 =
=
= f2 ,
˜b0 y
f0 f1

f˜1 = −˜b1 z = −b0 z = f0 ,

hence, the action bi → ˜bi and fi → f˜i is σ0 . Similarly, if we consider the element that changes
correspondences between (11) and (14), we have that ˜bi = bi and c˜ = 1/qc, or more precisely,
q˜
c2 = 1/qc2 . The transformations of the fi are given by
1
a1
= ,
f˜0 =
˜b0 y
f0

1
f˜1 = −˜b1 z = .
f1

The transformation, bi → ˜bi and fi → f˜i is σ1 . This adds additional structure to the lattices
found in previous studies [23, 22].
An interesting secondary consequence is that the above table also presents us with a way
in which we may reduce the dimension of the lattice. If there is to be a full correspondence

12

C.M. Ormerod

between the connection preserving deformations and the root lattice of type (A2 + A1 )(1) then
the composition of the connection preserving deformations that represent T0 ◦ T1 ◦ T2 would be
a trivial transformation. Indeed we find that




κ1 κ 2 a 1 κ 2 w y
κ 1 κ2 a 1
−1
−1
Ta1 ,λ1 ◦ Tκ1 ,λ1 ◦ Ta2 ,λ1 :
: w, y, z →
:
, ,z ,
λ1 λ2 a 2
λ1 λ2 a 2
κ1 q
which corresponds to the identity element in each of the cases presented in Table 1. Hence, we
may write
hT0 , T1 , T2 , T3 i ∼
◦ Tκ1 ,λ1 ◦ Ta−1
i.
= L /hTκ1 ,κ2 ,λ1 ,λ2 , Ta−1
1 ,λ1
2 ,λ1
This quotient could be removed by appropriately fixing or removing two redundant variables,
which may allow a more explicit correspondence between the variables associated with the linear
problem and the fi ’s and bi ’s.
Using (11), we find that the symmetry, Sa1 ,a2 is equivalent to the symmetry




b1 f 1 + 1 ˜
b0 b1 b11
b0 b1
, f1 ,
s1 :
f 0 , f1 , f 2 →
f0
b2
b2 b1
b1 + f 1
while using (12) we find s0 and by using (13) we find s2 .

6

Conclusion

We have that there is additional structure in the lattice of connection preserving deformations
as various directions on the lattice present the same evolution equations. By considering the
fact that several of the translations on the lattice of connection preserving deformations present
equivalent evolution equations, we recover an automorphism group that permutes various directions on the lattice, which corresponds to the group of Dynkin diagram automorphisms. These
automorphisms could be incorporated into all the lattices found in the previous study [23].
Using these automorphisms, and the one known symmetry, we also recover the symmetries, giving the full set of B¨acklund transformations from the standpoint of the connection preserving
deformations.

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